Image of tunicate colonies of the genus Didemnum (probably the species D. vexillum) advancing from left to right over pebble gravel habitat taken November 2003 on northern Georges Bank. Water depth was 41 meters or 135 feet. Photo/USGS.

Phil Colarusso, a diver with the Environmental Protection Agency, writes in the EPA blog about the issue of invasive tunicates known as sea squirts and their increasing impact on the aquatic ecosystem of the Northeast.

As a diver for EPA, I am often asked, “what’s the scariest thing you’ve ever seen underwater?” Most people expect the response to be sharks or stingrays. They are generally disappointed and perplexed when I say sea squirts.

Let me explain. Sea squirts, also known as “tunicates,” are seemingly benign-looking gelatinous, filter-feeding animals. They grow in large colonies or as individuals. They don’t bite, sting or even generally move, once they have attached themselves to a solid object, like a rock or pier.

Tunicate, photo/USGS.

So why the concern? Well, here in the northeast, like many other places in the U.S., we’ve been experiencing an invasion of non-native tunicates. Mooring lines, piers, boat bottoms, rocks, shellfish, and even eelgrass have been overgrown by various invasive sea squirts. Anything that does not move quickly is at risk of being engulfed by the rapidly reproducing and growing colonial species of sea squirts.

Sea squirts are prolific filter feeders, processing more water per unit body weight than oysters, blue mussels or quahogs. Very few animals feed directly on them because they are not a native species.

Our dive unit has been studying the impact of these invasive species on the ecology of coastal salt ponds on Martha’s Vineyard. We initially became interested in this topic while doing eelgrass restoration work, when we noticed large numbers of eelgrass shoots covered with exotic sea squirts.

Read more:

EPA Blog: Looking for Sea Monsters

For a video from the USGS, CLICK HERE (2004).

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